(Reuters) - Migrants separated from their children after they were detained for illegally entering the United States filed a class action lawsuit on Friday, claiming there are "hundreds" of parents in the same situation, and that the Trump administration is violating their due process rights.

The lawsuit, brought by the American Civil Liberties Union and filed in U.S. District Court in southern California, expands on the claim of a single Congolese asylum seeker filed last week. Ms. L, as she is referred to in the complaint, had been detained in San Diego while her 7-year-old daughter was sent to Chicago four months ago by federal authorities. But on Tuesday, just days after the initial lawsuit was filed, Ms. L was released.

The class action seeks to represent all adult parents in immigration custody who have a minor child separated from them without a hearing to prove the parent is unfit to care for them.

The Justice Department declined to comment on pending litigation.

The Department of Homeland Security (DHS) said that it does not currently have a policy of separating families but that it does so at times if a child may be at risk.

"There have been numerous intelligence reports and cases where kids have been used and trafficked by unrelated adults in an effort to avoid detention," Tyler Houlton, a DHS spokesman, said in a statement. "If we are unable to confirm this relationship we must take steps to protect the child."

In the class action complaint, Ms. L is joined by another anonymous lead plaintiff. Ms. C. is a woman from Brazil who crossed the border and told officials she hoped to apply for asylum in the United States. She was prosecuted on a misdemeanor charge of entering the country illegally and served a 25-day sentence in jail, while her 14-year-old son was sent to a facility in Chicago, the lawsuit says. On Sept. 22, she was transferred to immigration detention in Texas and has still not been able to reunite with her son.

The administration has floated the idea of separating families at the border in an attempt to deter migration.

But Democratic lawmakers have spoken out against that idea and the American Academy of Pediatrics said in a statement that family separation measures "are harsh and counterproductive."

(Reporting by Mica Rosenberg in New York; Editing by Sue Horton, Susan Thomas and Lisa Shumaker)